Count,
There's quite a difference in the actions of the rods you've listed. All great rods but probably depends on your casting style and how fast or slow you like your rods to be. I've been fishing the SLT for four years and like it alot. It fits my casting stroke well and is great fun fighting fish with. I have cast the Z-Axis but never fished them. I do like the Z action in the lighter rods but not in the heavier rods, just my preference. I'd certainly suggest you spend some time casting each one before deciding.
Scott

I've fished both and was blown away by how good both rods are, but for different applications.

I fished the SLT on the Yellowstone during the Caddis hatch this year and was tossing dries with it. It's a stunning Dry/Nymph rod. I absolutely fell in love with it and have been considering getting a 4 or 5wt as my dry/nymph/spring creek/no wind rod. To me the medium action in a heavier weight would be two slow for anything but lighter flies.

The Z-Axis is my kinda rod though. Like Scottflycst stated above, the Z-Axis is better in the lighter applications rather than the heavier. I fished the 691-4 Z-Axis on the Lower Madison, chucking two streamers. It launched those bugs like no tomorrow. This is going to be my go-to-rod because I fish big stuff 90% of the time and it has enough grunt to punch those bugs into the wind. I currently have this rod on order.

A few weeks ago I was able to cast the 6 and 7wt Z-Axis rods side by side during a demo day. It was amazing to me how they could cast like two totally different rods. I much preferred the 6 to the 7.